1. Field of the Invention
The present invention relates to an apparatus which performs, by optical CDMA (Optical Code Division Multiplex Access), at least one of encoding and decoding of wavelength-division-multiplexed light. More particularly, the present invention relates to an apparatus which employs optical fiber gratings to perform encoding/decoding by optical CDMA.
2. Description of the Related Art
In optical CDMA, a technique similar to the CDMA technology which has been practically used in the field of mobile communications is employed to perform encoding of an optical signal at a transmitting end, and decoding of an optical signal at a receiving end. The encoding/decoding of an optical signal is performed by using optical devices such as diffraction gratings, optical waveguides, or optical fiber gratings.
In optical CDMA, even if a number of encoded optical signals exist in the same wavelength band, interferences therebetween are prevented because of code-by-code independence. Therefore, by assigning different codes to different users, it becomes possible for a large number of users to simultaneously share one optical signal propagating medium, even though optical signals in the same wavelength band are used.
Currently proposed encoding methods can be classified into, for example: Frequency-encoding techniques; Frequency-Hopping techniques; Fast-Frequency-Hopping techniques; and direct-sequence techniques. A Frequency-encoding technique is a method of encoding which varies the intensities of optical signals for different wavelengths. A Frequency-Hopping technique and a Fast-Frequency-Hopping technique are methods of encoding which vary wavelength and delay. A direct-sequence technique is a method of encoding which varies delay and phase for a single wavelength.
An apparatus which employs optical fiber gratings to perform encoding by applying delays of sizes which are in accordance with the respective wavelengths to a light pulse signal is disclosed in: “Passive Optical Fast Frequency-Hop CDMA Communications System” Habib Fathallah, Journal of Lightwave Technology, Vol. 17, No. 3, March 1999 (hereinafter “Non-patent Document 1”); and “Robust Optical FFH-CDMA Communications: Coding in Place of Frequency and Temperature Controls” Habib Fathallah, Journal of Lightwave Technology, Vol. 17, No. 8, August 1999 (hereinafter “Non-patent Document 2”).
FIG. 1 is a diagram corresponding to FIG. 1(b) of Non-patent Document 1, showing the structure of an encoder which encodes an incoming pulse. This apparatus comprises a plurality of optical fiber gratings of the same structure, each having a uniform grating. A piezoelectric device is attached to each optical fiber grating, so that different tensions can be applied to the respective optical fiber gratings.
The wavelength of reflection (hereinafter “reflection wavelength”) by each optical fiber grating can be shifted by adjusting the magnitude of the tension applied thereto. As a result, light components of different wavelengths that compose the incoming pulse can be selectively reflected.
Since the optical fiber gratings are present at different positions within an optical fiber, the reflected light components will have various optical path differences, i.e., different delays. Therefore, the encoding code combination is reflected in the delay pattern.
Based on a specific combination of selected wavelengths and a combination of delays differing for each wavelength, a code pattern under the FFH-CDMA (Fast Frequency Hopping-Code Division Multiplexing Access) technique can be defined. Such a code pattern can be expressed in a matrix as shown in FIG. 2.
By controlling the magnitudes of the tensions applied to the optical fiber by means of e.g. piezoelectric devices, the reflection wavelengths can be varied, thus shifting the wavelengths which will receive specific delays. By setting the wavelengths to receive specific delays to arbitrary values, the code pattern of an encoder can be programmed. On the other hand, a decoder applies opposite delays to light of wavelengths that have been used for encoding. In other words, the order of the gratings used in the optical fiber of a decoder is a reverse of the order of the gratings used in the optical fiber of an encoder.
Hereinafter, with respect to FIGS. 2 to 5, the encoding/decoding principle used in optical CDMA will be described more specifically.
FIG. 2 shows a 3×3 matrix (Frequency-Hop pattern) corresponding to a certain code pattern. In this matrix, the horizontal axis represents time, whereas the vertical axis represents wavelength. The black blocks (elements) are time bins to which corresponding wavelengths are assigned. In the code pattern of FIG. 2, wavelengths λ1, λ2 and λ3 are assigned to time bins t1, t2 and t3, respectively.
FIG. 3 schematically shows: (left side of the arrow) a light pulse sequence expressing a code which has been encoded according to the code pattern of FIG. 2; and (right side of the arrow) a light pulse whose code has been decoded. In FIG. 3, “encoding” corresponds to a conversion from the right side to the left side, whereas “decoding” corresponds to a conversion from the left side to the right side.
FIG. 4 shows an encoder which performs the encoding as shown in FIG. 3.
In the encoder shown in FIG. 4, an optical fiber in which three gratings 3, 4 and 5 are formed at a predetermined interval is coupled to optical fibers 1 and 6 via a circulator 2. A light pulse which has propagated through the optical fiber 1 passes through the circulator 2, and thereafter enters one end of the optical fiber in which the gratings 3, 4 and 5 are formed.
A light component of the wavelength λ1 contained in this light pulse is reflected by the grating 3, and thereafter passes through the circulator 2 to enter the optical fiber 6. On the other hand, any light other than the wavelength λ1 that is contained in the light pulse is transmitted through the grating 3. Out of the light pulse having been transmitted through the grating 3, a light component of the wavelength λ2 is reflected by the grating 4, and thereafter passes through the circulator 2 to enter the optical fiber 6. Any light component other than the wavelength λ2 is transmitted through the grating 4. Out of this transmitted light, a light component of the wavelength λ3 is reflected by the grating 5, and thereafter passes through the circulator 2 to enter the optical fiber 6.
Thus, since an optical signal that has propagated through the optical fiber 1 in the form of a single light pulse is reflected by the gratings which are disposed at different positions corresponding to different wavelengths, the light pulse is separated into three light pulses on the time axis, which sequentially enter the optical fiber 6. In accordance with the structure of FIG. 4, light pulses of the wavelengths λ1, λ2 and λ3 will enter the optical fiber 6 in this order; however, by changing the order in which the gratings 3, 4 and 5 are arranged, the order of the light pulses entering the optical fiber 6 can be changed. By changing the order in which the gratings 3, 4 and 5 are arranged, it becomes possible to perform encoding by different code patterns.
Axes 7, 8 and 9 shown in FIG. 4 represent the center positions of the three gratings 3, 4 and 5, respectively. The interval between the centers of the gratings 3 and 4 is the distance between the axes 7 and 8. The interval between the centers of the gratings 4 and 5 is the distance between the axes 8 and 9.
The optical path difference which exists between the light of the wavelength λ1 and the light of the wavelength λ2 is twice the interval between the centers of the grating 3 and the grating 4. Similarly, the optical path difference which exists between the light of the wavelength λ2 and the light of the wavelength λ3 is twice the interval between the centers of the grating 4 and the grating 5. Thus, the relative positioning of the gratings 3, 4 and 5 defines the time differences between the three time bins for encoding.
As shown in FIG. 4, assuming that the time (delay) required for light to propagate between the gratings 3 and 4 or between the gratings 4 and 5 is always ΔT, the relative delay between light of the wavelength λ1 and light of the wavelength λ2 and the relative delay between light of the wavelength λ2 and light of the wavelength λ3 are both 2ΔT, as shown in FIG. 3. In other words, the axes 7, 8 and 9 shown in FIG. 4 correspond to the time bins t1, t2 and t3, respectively, the three time bins t1, t2 and t3 being respectively assigned to the wavelengths λ1, λ2 and λ3.
As shown in e.g. FIG. 3, “encoding” in optical CDMA corresponds to: separating a single light pulse into a plurality of smaller pulses of light having the wavelengths λ1, λ2 and λ3; and outputting these light pulses with predetermined relative delays. On the other hand, “decoding” corresponds to reassembling the plurality of light pulses which have been separated on the time axis into a single light pulse. In order to perform decoding, it is necessary to cause the light pulses (wavelengths λ1, λ2 and λ3) to have opposite delays of the delays which were applied at the time of encoding, thus canceling the delays which occurred during encoding. In other words, it is necessary to synchronize the three small pulses to the same point in time, thus combining them into the same time bin. When three light pulses (wavelengths λ1, λ2 and λ3) which have arrived at different points in time are synchronized to the same point in time, and detected by a detector as indicating a light intensity which is equal to or greater than a threshold value, the decoder will recognize this signal as 1 bit (the right side of FIG. 3).
When encoding data for transmission (e.g., a bit sequence which is expressed as “11001001 . . . ”), the encoder sends out a light pulse sequence (wavelengths λ1, λ2 and λ3) indicating the “1” bit, and does not send out a light pulse sequence indicating the “0” bit, for example. In this case, only when the aforementioned light pulse sequence has entered a decoder which is reciprocal to (i.e., provides opposite delays of) the code pattern of the encoder, the decoder will detect a signal indicating the “1” bit.
FIG. 5 shows a decoder which performs the decoding illustrated in FIG. 3. The decoder structure of FIG. 5 differs from the encoder structure of FIG. 4 in terms of the order in which the reflection wavelengths of the gratings 10, 11 and 12 are arranged. Specifically, the reflection wavelengths of the gratings 10, 11 and 12 are set to be λ3, λ2 and λ1, respectively.
In an encoder having the above structure, if the reflection wavelengths of the gratings are changed due to changes in temperature or application of tension, a so-called “wavelength shift (wavelength drift)” may occur, which makes correct encoding impossible, or in the case of a decoder, makes correct decoding impossible.
A wavelength shift or wavelength drift refers to a deviation in the reflection wavelength band of a grating from its setting value, which may occur due to expansion of a grating in the encoder/decoder in response to a change in the ambient temperature, etc., or due to changes in the refractive index of the grating. In general, as the grating temperature increases, or as a greater tension is applied to the grating, the reflection wavelength band of the grating is more shifted toward the longer wavelength.
Since a code pattern is defined by a combination of a predetermined plurality of reflection wavelengths, a “wavelength shift” can cause a fatal error during encoding or decoding. For example, if a “wavelength shift” occurs in an encoder, the encoder will perform encoding by using wavelengths which are different from the wavelengths that define the intended code, thus making it impossible for the reciprocal decoder to perform decoding. Conversely, if a “wavelength shift” occurs in a decoder in the case where encoding has been performed correctly, it becomes impossible to correctly decode the code. These will induce data transmission errors or transmission failures. In order to prevent such a “wavelength shift”, the gratings of an encoder/decoder are to be maintained at a constant temperature.
Non-patent Document 2 discloses a technique which prevents problems in encoding/decoding even in the presence of a slight “wavelength shift”, this technique being based on the Fast-Frequency-Hopping technique. According to this technique, the number of chips in the code pattern is reduced to give a large margin to the band of each chip, whereby some immunity to non-wavelength-dependent shifts is obtained.
As used herein, a “non-wavelength-dependent shift” means a wavelength shift where the shift amount does not depend on the wavelength. On the other hand, a “wavelength-dependent shift” means a wavelength shift where the shift amount differs depending on the wavelength. A wavelength-dependent shift occurs in the case where different temperatures or different tensions are applied to the individual gratings.
Even when using the code pattern described in Non-patent Document 1, if the magnitude of the wavelength shift differs depending on the wavelength, most of the light components contained in the signal pulse light will suffer a loss and disappear. Moreover, as long as a single uniform grating is used for each chip of the code pattern, it will be difficult to provide each reflection wavelength band with a large width.
A non-wavelength-dependent shift represents a special case of a wavelength-dependent shift. That is, when the shift amounts for all wavelengths are equal in a wavelength-dependent shift, there exists a non-wavelength-dependent shift. Therefore, a non-wavelength-dependent shift is encompassed under the broad definition of a wavelength-dependent shift. This means that more stringent conditions will need to be satisfied to obtain immunity against wavelength-dependent shifts than to obtain immunity against non-wavelength-dependent shifts.
In order to overcome the problems described above, a main purpose of the present invention is to enhance immunity against wavelength-dependent shifts in an encoder and decoder compliant with optical CDMA.